1. Field of the Invention
The present invention relates to an image processing method of generating conversion data for a scanner, and to a calibration method of calibrating a printing apparatus by means of the scanner.
2. Description of the Prior Art
It is generally known that a printing property of a printing apparatus may vary according to a surrounding condition such as a temperature and a humidity of a surrounding air, in which the printing apparatus is used. Not only the surrounding condition but also use of the printing apparatus for a certain period of time may cause the printing property to vary. In a case of electro-photographic printing apparatus, for instance, a photosensitive property of a photosensitive drum changes according to the surrounding condition and to aging by the use of the apparatus. As a result, the printing property e.g. a gradation in density observed in a printed image changes from what is desired. It is also known that in a case of an ink-jet printing apparatus, the above-mentioned printing property change is caused by a change in ejection property of a print head, for instance.
A calibration is carried out to deal with such change in the printing property but is not carried only for the change in printing property of an individual printing apparatus as described above. In a case of an information processing system in which a plurality of printing apparatuses are connected with one another through a network, difference in the printing property among the plurality of printing apparatus may present a problem. Thus, also in such a case, it is necessary to conduct the calibration to reduce the printing property variations among the printing apparatus.
A conventional calibration is basically carried out based on the user's instruction input. For example, when the user observes that the gradation in the printed image is not what is desired, the user gives an instruction to execute the calibration on an operation picture of a display and others of the printing apparatus or a personal computer (hereinafter also referred as a PC).
In order to carry out the calibration, the user causes the printing apparatus to print a predetermined chart in which patches of a plurality of density for each color are disposed, and causes a densitometer, a scanner or others to read the printed chart. Then, based on read data, the user makes, for example, contents of a density correction table used in the image processing for generating printing data renewed. Accordingly, in order to carry out the calibration of the printing apparatus with high accuracy, it is also important to read the chart printed by the printing apparatus correctly.
As a device for reading the chart, there is known the densitometer. The densitometer gives so-called device-independent data such as CIE-1931XYZ color system data, L*a*b* system data or the like (hereinafter the data is called “density”) as a result of measuring the chart. This allows the density obtained by measuring the chart to correctly reflect the printing property of the printing apparatus which prints the chart, irrespective of the device used for measurement. The densitometer, however, is relatively expensive; it is not realistic to supply a printer with an expensive densitometer as peripheral equipment, or to make the user purchase the densitometer separately in order to calibrate a low-price printer.
On the other hand, in order to read the chart, a scanner which is a device to input an image and others, or a reading portion of a copying machine have been used, conventionally. However, it is difficult to use the scanner and others as a highly accurate device for reading the chart, as the densitometer. This is because conventionally a calibration for the scanner and others has not been done so as to be able to conduct the highly accurate density measurement. More specifically, it is relatively difficult to generate a table for converting brightness data obtained by the scanner into the density (hereinafter the conversion is called “brightness-density conversion”) with high accuracy.
In addition, when the calibration of the scanner and others can not be carried out with high accuracy, different types of scanners and others may become to have different properties. Then, when such scanners are used for the calibration of the printer, different colors may be reproduced among printers.